20 BULLETIN 1018, U. S. DEPARTMENT OF AGRICULTURE. 



gators (Klebs, 16, and Appleman, 2) in being able to maintain almost 

 any plant in continuous growth throughout the entire year would 

 seem to indicate that the importance of periodicity has been exag- 

 gerated. 



It is difficult to interpret the behavior of Pima cotton as above 

 described on the basis of an inherent periodicity, but it is not im- 

 probable that physiological reactions are involved. It is a common 

 observation among growers that cotton plants once stunted by water 

 shortage are much slower in response when water is finally applied 

 than plants which have not been so severely checked. A case of 

 this nature came under the writer's observation at the Sacaton sta- 

 tion in 1920. Plants growing in a sandy soil showed visible distress 

 from water shortage about two weeks before the first application of 

 water on July 7. In response to this irrigation the plants grew 

 only 9 inches in height during the ensuing three weeks and re- 

 quired two additional irrigations before complete recovery, while 

 plants on adjoining plats, which had shown no water stress, grew 

 14 inches during the three weeks following the first irrigation and 

 responded promptly to each subsequent irrigation by making addi- 

 tional growth. The fact that all external factors except soil moisture 

 were closely alike for all the plants in these experiments and that 

 there was a distinct difference in the promptness of response to this 

 particular factor gives probability to the interpretation that the 

 cause is internal, but is subject to remedy by control of the external 

 factors. 



When we come to consider the adoption of means for remedying 

 this maladjustment of the plant to its water supply, the most prac- 

 tical line along which to work seems to be in modifying the condi- 

 tions of the subterranean environment. The need for some such 

 corrective measure seems especially imperative when dealing with 

 heavy rich soils which cause a quick and vigorous response of the 

 plants to irrigation water when applied by conventional methods. 



An experiment was conducted in 1919 by the writer and his co- 

 operator, Mr. Warren H. Parker, in adjacent plats to those above 

 described, in the effort to effect more perfect control of the plant 

 growth. 4 The plants were grown in pairs of rows, with a wide 

 middle 5-| feet between pairs and a space of 3| feet between rows. 

 The water was run in deep furrows made in the wide middle (PI. IV) , 

 while the narrow middles were left dry after the planting season. 

 The effect of the water when applied under these conditions was not 

 rioted in the behavior of the plants until five or six days later, and 



4 A double-row method of planting and irrigating Egyptian cotton has been outlined in 

 connection with the single-stalk method o£ suppressing the vegetative branches, and 

 tested in recent seasons at the United States Experiment Farm at Bard, Calif., near 

 Yuma, Ariz. (6, 7, 8, 21). 



